The field of consumer electronics places a high value on minimizing size and improving portability, particularly in wireless communication devices. The need for an adequately long antenna, however, limits how small certain wireless devices can be. Antenna efficiency is a function of many parameters, including an antenna's length. Generally, most receivers function well enough with antennas half the wavelength or one quarter of the wavelength of the signal being received. Receivers using antennas substantially less than one quarter of the wavelength, however, will have less adequate reception.
The wavelength (λ) of a signal equals the speed of light (c) divided by the frequency (f). For example, 2.4 GHz signals, such as those used by Bluetooth devices, cordless phones, wireless routers, and other household devices have wavelengths less than 13 centimeters. FM radio signals, which range from approximately 87 MHz to 108 MHz, have wavelengths from 277 centimeters to 344 centimeters.
A λ/4 antenna for a 2.4 GHz headset only needs to be about 3 cm, compared to about 86 centimeters for a headset receiving radio waves. A high frequency device such as a wireless headset for a cell phone can, therefore, still be quite small and have an antenna capable of good reception. Receiving lower frequency signals such as radio waves on that same headset, however, would be quite challenging. Most typical handheld radios overcome these limitations by either using an extendable metal antenna or by using the radio's headphone cords as an antenna. These two solutions, however, are both less than ideal because they both greatly increase the physical size of the system.
It would be desirable to build a small device capable of receiving lower frequency signals without the need for bulky external antennas.